Energy storage is ubiquitous in daily life. For example, various batteries can be found in mobile electronic devices, automobiles, or utility-scale backup systems. Also, energy storage is becoming more important as energy generation transforms into a renewable-focused structure, at least in part because of the intermittent nature of renewable energy sources.
Electrochemical cells are energy storage devices in which energy from reduction-oxidation (redox) chemical reactions are converted to electrical energy. An electrochemical cell generally includes a cathode, an anode, and an electrolyte. Reduction and oxidation reactions occur at the cathode and anode, respectively, while charged ions are transported through the electrolyte to generate an electrical current. In a “primary cell” (e.g., a non-rechargeable battery), the electrochemical reactions are irreversible. As the stored electrochemical energy is exhausted, the battery can no longer be used. Redox reactions in a “secondary cell” are reversible, and such batteries can be referred to generally as “rechargeable.”
The lead acid battery, invented by Gaston Planté in 1859, can be regarded as the oldest type of rechargeable battery. Despite having a relatively low energy density as compared to other chemistries, generally-available lead acid batteries are simple in construction and economical. Such generally-available lead acid batteries are used in automotive, traction, and stationary applications such as for ignition or for starting internal combustion engines; for lighting; for applications such as motorized wheelchairs, golf carts or fork lifts; or for other applications such as electrical energy storage when coupled to the electric grid.